The present invention relates to the field of design and testing of artillery shells, and more particularly to the recovery of artillery shells that have been fire from guns.
In the art of design and testing of artillery shells, and in stockpile evaluation, especially for atomic artillery shells, numerous test firings are conducted. In many of the test firings, it is necessary to recover the shell intact to evaluate the performance of its components. In flight, the shells are spin stabilized and can be revolving at 10,000 to 20,000 rpm. To permit recovery, the shells are fired from guns set at high elevation angles, about 85-88 degrees above the horizontal. On such high trajectories, the shell falls base first after passing through the apogee of its trajectory. A parachute recovery system [Pepper, W. B. and Fefferhoff, R. D., "Parachute System to Recover Spinning (250 RPS) 155-mm Shell Subjected to 20,000 g's Launch Conditions," Sandia Report SC-R-68-1806, September 1968 AIAA Paper No. 68-937 of American Institute of Aeronautics and Astronautics] is deployed from the nose sometime after the apogee is passed. While the shell is suspended on the parachute and still spinning at a high rate, its aerodynamic and gyrodynamic characteristics can cause it to nutate from a base down to a nose down attitude.
Recovery system failure can result from a number of causes. For example, the parachute and suspension lines may become tangled during deployment at the high spin rates. Or, the swivel for the suspension line may fail. If the parachute does deploy properly and the rapidly spinning shell nutates to a sufficient angle, the shell nose can rub on the support lines to the parachute. The suspension lines may be worn into two by the shell's rubbing and also cause a recovery system failure.
When the recovery system fails, the shell can free fall to the ground and be severely damaged. Although the present recovery system provides better than an 85% success rate, it would be desirable to improve recovery system reliability. More specifically, the cost of each test for a shell is presently between $100,000 and $250,000. Even a 15% failure rate leads to much unwanted cost.
In the past, attempts have been made to despin test shells. In one previous attempt, the shell recovery system requires that the shell rotate into a nose down attitude so the shell will rub against the parachute suspension cable to produce a despin torque. As mentioned above, such a system may directly lead to failure of shell recovery.
In another past attempt to despin test shells, several rings were mounted in the shell's nose concentric with its longitudinal (spinning) axis. The rings were free to move both radially and longitudinally and to revolve relative to the shell. The presence of the rings was designed to cause the shell to become unstable after gun launch and to change the spin about the longitudinal axis into a large coning motion which would produce spin deceleration through aerodynamic damping. A severe problem with the ring system was the occurrence of severe vibrations in the shell that resulted in damage and partial destruction of shell components. Consequently, the ring system was abandoned. It would be desirable, therefore, to provide a shell recovery system which did not result in destruction of shell components.